In a time of flight mass spectrometer (TOF-MS), ions accelerated by an electric field are injected into a flight space where no electric field or magnetic field is present. The ions are separated by their mass numbers according to the flight time until they reach a detector and are detected thereby. Since the difference of the lengths of flight time of two ions having different mass numbers is larger as the flight path is longer, it is preferable to design the flight path as long as possible in order to enhance the mass number resolution of a TOF-MS.
In many cases, however, it is difficult to incorporate a long straight path in a TOF-MS due to the limited overall size, so that various measures have been taken to effectively lengthen the flight length. In the Japanese Unexamined Patent Publication No. H11-195398 (called “Patent Document 1” hereinafter), an “8” shaped orbit is formed using two or four sector-shaped electric fields, and the ions are guided to fly repeatedly in the “8” shaped orbit many times, whereby the effective flight length is elongated.
In general, the time-focusing and space-focusing of ions are important for a TOF-MS to perform analyses with high accuracy, as pointed out in Patent Document 1 or by Ishihara et al. (“Perfect space and time focusing ion optics for multiturn time of flight mass spectrometers”, International Journal of Mass Spectrometry, 197(2000), pp. 179-189). It is said that, even if the ions leave the same position into different directions with different levels of energy, they can simultaneously reach the same position as long as they satisfy the aforementioned two focusing conditions, although they differ in flight direction and energy level. In actual analyses, however, the space-focusing condition does not need to be very tight if the object of the analysis is to measure the ion strength with respect to the mass number of the ion. This is because the ion detector, whose detecting surface has a certain area, is able to detect the ions even if they do not reach the same position on the detecting surface. Therefore, time-focusing is more important.
Patent Document 1 claims that the ion optics constituting the loop orbit in the TOF-MS described therein is capable of achieving the time-focusing of ions by disposing sector-shaped electric fields in double symmetry. This configuration attempts the time-focusing of ions within the multiple loop orbit, whereas it gives no consideration to the flight path along which the ions released from the ion source travel until they enter the multiple loop orbit or the flight path along which the ions that have flown the multiple loop orbit predetermined times and left the multiple loop orbit travel until they reach the ion detector. Thus, the analysis cannot always be carried out with adequate accuracy.
The main object of the present invention is therefore to provide a time of flight mass spectrometer capable of creating an improved mass spectrum and calculating the mass number of each ion from the-spectrum with high accuracy.